Solid fuels

ABSTRACT

In order to provide a block (1) of solid fuel material which has good burning time, good flame form and a clean flame, the block (1) is comprised of a mixture of solid particulate, cellulosic, organic combustible material, and stearic or palmitic acid together with, optionally, one or more fatty acids or esters. The mixture has a density of from 0.94 to 1.01 gm/cc. The block (1) has a slit (2) extending longitudinally of the block. The slot (2) communicates with a space (3). Combustible gases collect in the space (3) and form a flame at the mouth of the slit (2).

This invention is concerned with improvements in and relating to solidfuel materials and the production thereof. The products are useful asfirelighters but may also be used as more conventional fuel materials.

Basically, in accordance with a first aspect of the present invention, asolid fuel material comprises a mixture of

(i) a solid particulate, cellulosic, organic combustible material, and

(ii) stearic and/or palmitic acid together with, optionally, one or moreother fatty acids or esters; the mixture having a density of from 0.94to 1.01 gm/cc, preferably 0.95 to 1.00 gm/cc, especially about 0.96 to0.99 gm/cc.

As noted above, the fuel materials of the first aspect of the inventionare composed of two principal ingredients, namely a particulate,cellulosic combustible material and a fatty acid component which servesas binder in the product of the invention and also as a fuel. The firstingredient which may for convenience be termed a "combustible filler",may be any of a wide variety of materials. Examples of suitable organicfillers include forestry residues, sawdust, wood shavings, straw,granulated or comminuted peat, shredded paper, bagasse and likematerials.

The particle size of the combustible filler may vary within wide limitsand will, to some extent, depend upon the nature of the combustiblefiller. The particulate organic material is preferably of a particlesize such that the material can pass through a 4 mm mesh sieve,preferably a 3 mm mesh sieve. As discussed in more detail below, themoisture content of the combustible filler can be of importance when apreferred method of production of the fuel is employed.

The second basic ingredient (hereinafter referred to as "the fatty acidcomponent") of the fuel material of the first aspect of the invention isone comprising stearic and/or palmitic acid, optionally with other fattyacids or esters. By the terms "stearic acid" and "palmitic acid", weintend, of course, to refer to commercially available materials soldunder these names or as "stearine" and which, in practice, comprisemixture of fatty acids in which the nominal acid predominates.

The ratio of combustible filler to fatty acid component should be suchthat there is sufficient fatty acid present to form a cohesive bindermatrix and, to this end, the fatty acid component is preferably presentin an amount such that it forms from 40 to 60%, preferably 45 to 55%, ofthe total weight of the finished product. In addition to serving as abinder the fatty acid component is, of course, also combustible and hasbeen found to give good burning characteristics, especially as comparedwith a mineral wax.

Other components may be present in minor amounts in the fuels producedin accordance with the first aspect of the invention such as perfumes orother odoriferous agents (e.g. to impart a "pine" smell to the product)or colouring agents. In particular, it has been found that theincorporation of a small amount of a starch (e.g. corn starch) orsimilar particulate binder provides not only for a tough, well-bondedproduct but also gives rise to a product having better burningcharacteristics, i.e. give a fuel block which burns longer with lowerflame (the proportion of fatty acid being the same) and give a coherentcalcined residue. Such binder is suitably present in an amount of from 2to 10% by weight of the total components of the fuel, especially about5% by weight thereof.

A further useful minor component of the fuels of the first aspect of theinvention comprises a fire retardant. Such fire retardants must not, ofcourse, be present in amounts sufficient to prevent combustion of thefuel but, when present at low levels below full fire retardant amounts,it has been found that their presence increases the overall burn timewithout otherwise adversely affecting the performance of the product,eg. as a firelighter or fire log. In particular, it has been found thatwhen used in combination with a binder as discussed above, fireretardants give products which burn to give glowing coals suitable foruse, for example, in barbeque applications. Suitable fire retardantmaterials include inorganic materials such as monobasic ammoniumphosphate, dibasic ammonium phosphate and boric acid; and organicmaterials such as chlorinated paraffins, e.g. those sold under the tradename "Cereclor" by ICI. The amount of fire retardant should, as notedabove, be not such as to prevent combustion of the fuel and it is amatter of simple experiment to establish suitable levels of anyparticular fire retardant. In the use of the ammonium phosphatematerials it has been found that levels of up to 2% by weight, eg. 1 to2% by weight, are suitable.

It has been found important the fuel material of the invention shouldhave a density of from 0.94 to 1.01 gm/cc, in order to impart goodburning, especially good flame, characteristics. To this end, it hasbeen found necessary to employ a process of manufacture that involves acompression step. Accordingly, in principle, fuel materials inaccordance with the first aspect of the invention are prepared by thesteps of forming a mixture of the combustible filler and binder and thencompressing the mixture to give a product of the desired density. Inpractice it has been found convenient to soften or melt the binderbefore the compression step and, in this case, the binder may be mixedwith the filler and the whole mixture then heated to melt or soften thebinder or, alternatively, the filler may be added to a previously moltenor softened binder. In either case the blend of binder and filler isthen thoroughly mixed and the mixture then compressed into product ofthe desired size and shape and, of course, density. Such final shapingmay be effected, for example, by direct compression or by extrusion,preferably the former. The pressure required to achieve the desireddensity from a molten or softened starting mixture is not too high, e.g.from 40 to 110 psi, preferably from 80 to 100 psi.

Where the above melting process using molten or softened binder isemployed, we have found that the moisture content of the particulatefiller can be of importance in obtaining a good product. Whilst we donot wish to be bound by any theory, it appears that the total volume of"liquid", i.e. moisture in the combustible filler and molten or softenedbinder, should be within certain limits, typically 60-70% of the totalmixture. To this end, when using, as is preferred, about 50% by weightof binder, the combustible filler should preferably have a moisturecontent of about 20-35% by weight, especially about 30% by weight. Thus,contrary to what might have been supposed, the presence of some moisturein the combustible filler is of advantage.

The solid fuel material may be formed into any suitable product, such asa firelighter, firelog or barbeque fuel block.

The final product may be formed by any suitable process into anysuitable shape. For example, it could be cast, extruded or pressed in amould. It is preferably extruded. The final product may take the form ofindividual units or, more preferably, blocks provided with weakeninglines to facilitate separation into individual units. It may be formedinto prismatic shapes of any suitable cross section, such a circle,square, rectangle or other shape.

Preferably, the cross sectional shape gives a high external surface areato volume ratio (being preferably in the range 0.05 mm ⁻¹ to 0.5 mm⁻¹,preferably around 0.1 mm⁻¹ ). The cross sectional area may havereentrant features in its profile. For example, it may be provided withconcave or "scalloped" sections. This provides a high surface area tovolume ratio and also allows the blocks to stack stably on one anotherand on other fuel without slipping. A high surface area to volume ratioallows a relatively high rate of burning and/or higher flames than wouldotherwise be possible.

The inventors have discovered that particularly good flame form andheating effect can be obtained by the provision of a slit in the solidfuel material block.

According to a second aspect of the invention, there is provided a blockof solid fuel material comprising a mixture of solid particulatecellulosic organic combustible material and a binder, wherein at leastone slit is formed in the surface of the block.

The slit is believed to provide a surface over or from which evaporationor transport of combustible gases (such as carbonisation products of thecellulosic material or vaporised binder) can occur, the combustiblegases passing along the slit to feed a flame. Whilst it is not intendedto limit the scope of the invention to the above explanation, the secondaspect extends to forms of slit deep and/or wide enough to provide asubstantially larger flame than would otherwise be possible. In effect,it is believed that the slit collects combustible material and "pipes"it to the outside of the block. Such combustible gas might otherwise betrapped inside the block and diffuse out slowly, thus giving small flamesize.

The term "slit" is used to cover any relatively narrow and deepindentation in the surface of the block, including bore holes, steepsided V-cuts, generally parallel side cuts etc. Preferably, the depth ofthe slit is greater than its minimum width at the surface of the block,being preferably around 2-4, more preferably around 3 times deeper thanit is wide.

A plurality of such slits may be provided extending over one or severalsurfaces or surface areas of the block.

The slot width is preferably in the range 3-20 mm, most preferably 5 mm.Where the block is extruded, a practical limitation on the smallest sizeof the slot is provided by strength of the extrusion die. The minimumslot width at the surface of the block is preferably in the range0.04-0.25, most preferably around 0.07 times the largest the dimensionof the block normal to the width of the slit.

Preferably, the slit extends in a plane generally normal to the surfaceof the block. It may have a depth in the region 10-50 mm, preferably 30mm. The slit may be in the region of 0.15-0.7, preferably 0.5 times themaximum depth of the block in the plane in which the slit extends.

It is particularly preferred that the slit should communicate with aspace of dimension greater than the width of the slit formed within theblock. It is found that such a space acts as a collection chamber forcombustible gases. The space is preferably of dimension in the range5-30 mm, preferably around 10-12 mm. It may be in the range 0.06-0.5times the maximum width of the block, preferably around 0.15 of thewidth of the block in the plane normal to the plane of the slit.Preferably, the space is located generally centrally in the block. Inorder to allow good collection of combustible gases and flame height,the maximum distance of any part of the block from the nearest surfaceof the space or outside surface of the block may be in the region 10-25mm, preferably around 15-17 mm. It may be in the region 0.15-0.4,preferably 0.25 times the maximum width of the block in the plane normalto the slit.

In a preferred embodiment, there is a longitudinally extending slit. Itmay communicate with a longitudinally extending space, preferably ofcircular cross section.

A longitudinally extending slit gives a pleasing and controllable shapeof flame extending over a large part of the block.

The slit may be configured to impart a substantial velocity tocombustible gases collected by the slit, to give a good form of flame.The area of the slit at the surface of the block may be about1/5th-1/20th, preferably 1/15th-1/12th of the total internal surfacearea of the slit and of any space with which the slit communicates inthe block.

The cross sectional area of the block may be in the range 1,000-12,000mm², preferably 3,000-10,000 mm², most preferably 5,000-8,000 mm².

Preferably, the ratio of the cross sectional area of the block in aplane normal to the slit to the cross sectional area of a spacecommunicating with a slit in the same plane is in the region 60:1 to10:1, preferably 30:1.

The fuel material produced in accordance with the invention is readilyignitable, for example by the application of a simple flame andmaintains a fierce flame when burning. Typically, a final product ofabout 40 gm will have an overall burning time of 18-20 min.

In practice, in a block provided with a slit in a first surface, inwhich the slit extends from a second surface which meets the firstsurface to a third surface which meets the first surface, it has beenfound that the flame may be made to exit the slit from whichever surfaceis uppermost by a draft effect preventing the flame exiting fromgenerally vertically orientated sides of the block.

The present invention will be further described with reference to thesingle figure which is a cross section of a solid fuel block accordingto the first and second aspects of the present invention.

The solid feel material block shown is an extruded block comprised of asolid particulate, cellulosic, organic combustible material and a binderaccording to the first aspect of the invention. The figure shows theprofile of the cross section of the block in a plane normal to theparallel sides of the extruded block.

According to a preferred form the second aspect of the invention, theblock 1 is provided with a slit 2 communicating with a circular sectionchamber 3 formed in the centre of the block.

The profile of block 1 is generally rectangular with rounded edges 4(which are less prone to damage than sharp edges). Scalloped indentions5 are provided in each side of the profile. These provide a high surfacearea to volume ratio for the block. They also allow the block to reststably on other blocks or fuel when placed in a fire, preventing theblock from rolling or falling.

When ignited, an unusually large flame is seen to emerge from the mouthof the slit 2.

In order to obtain the best effect, the block 1 should be placed withthe slit 2 pointing upwards or outwards so that the flame exiting theslit 2 can be seen. To this end, the block 1 may be provided withmarkings to identify its upper surface or in a wrapper which indicatesits upper surface.

In order that the invention may be well understood, the followingExamples are given by way of illustration only.

EXAMPLE Example 1

Forest wood residue was chipped to a particulate size such that allpassed through a 4 mm aperture sieve.

Fifty parts by weight of the chipped residue was mixed with fifty partsby weight of prilled stearic acid (nominal diameter-1 mm) and themixture heated to above the melting point of the stearic acid.

The whole was then thoroughly mixed and pressed in 400 gm lots in ahydraulic press under a pressure of about 100 psi. The resultantcompressed product had a smooth attractive appearance and burned with afierce flame.

Example 2

Following the above procedure, except that the mixture was pressed in425 gm lots, a fuel product was prepared from the following:

stearine 47.5% by wt

woodchip 47.5% by wt (30%) moisture content)

starch 5.0% by wt.

Example 3

Following the procedure of Example 2, a fuel product was prepared fromthe following:

woodchip (dry) 45% by wt.

palm stearine wax 45% by wt. dibasic ammonium

phosphate (20% aq) 10% by wt.

Example 4

Following the procedure of Example 2, a fuel product was prepared fromthe following:

woodchip (dry) 46% by wt.

palm stearine wax 45% by wt. dibasic ammonium

phosphate (20% aq) 5% by wt.

Corn starch 4% by wt.

We claim:
 1. A block of solid fuel material comprising a mixture of(i) asolid particulate, cellulosic, organic combustible material, and (ii) abinder, said binder containing stearic and/or palmitic acid togetherwith, optionally, one or more other fatty acids or esters;wherein atleast one slit is formed in a surface of said block, and wherein a depthof said slit is in a range of about 0.15 to 0.7 times a maximumthickness of said block measured parallel a plane along which said slitextends.
 2. The solid fuel material according to claim 1, the mixturehaving a density of from 0.94 to 1.01 gm/cc.
 3. The solid fuel materialaccording to claim 1, further comprising a starch or similar particulatebinder.
 4. The solid fuel material according to claim 1, furthercomprising a fire retardant.
 5. The solid fuel material according toclaim 4, wherein the fire retardant material comprises monobasicammonium phosphate, dibasic ammonium phosphate, boric acid, or achlorinated paraffin, or a mixture thereof.
 6. A method of preparing asolid fuel material according to claim 1, comprising the steps offorming a mixture of the combustible filler and binder and thencompressing the mixture to give a product density of from 0.99 to 1.0g/cc.
 7. The method according to claim 6, further comprising the step ofsoftening the binder before the compression step.
 8. The block accordingto claim 1, wherein the depth of the slit is 2 to 4 times the minimumwidth of the slit at the surface of the block.
 9. The block according toclaims 1 or 8, wherein the slit communicates with a space of dimensiongreater than the width of the slit formed within the block.
 10. Thesolid fuel material according to claim 2, the mixture having a densityof from about 0.95 to 1.01 gm/cc.
 11. The block according to claim 8,wherein the depth of the slit is 3 times the minimum width of the slitat the surface of the block.
 12. The solid fuel material according toclaim 1, wherein said stearic and/or palmitic acid and the one or moreoptional fatty acids or esters are present in an amount within a rangefrom about 40 to about 60 wt. %, based on the total weight of saidmaterial.
 13. The solid fuel material according to claim 12, whereinsaid stearic and/or palmitic acid and the one or more optional fattyacids or esters are present in an amount within a range from about 45 toabout 55 wt. %, based on the total weight of said material.